How do I find matching/nesting anything?

This isn't something that can be done in one regular expression, no matter how complicated. To find something between two single characters, a pattern like /x([^x]*)x/ will get the intervening bits in $1. For multiple ones, then something more like /alpha(.*?)omega/ would be needed. But none of these deals with nested patterns. For balanced expressions using (, {, [ or < as delimiters, use the CPAN module Regexp::Common, or see "(??{ code })" in perlre. For other cases, you'll have to write a parser.

That said, for your situation, you can repeatedly find the innermost expression and replace it using a dispatch table: